Scanning method using a rotating prism

Abstract

A scanning apparatus and method for scanning a beam of light incorporate an optical system in which a rotatable wheel has a plurality of light deflecting facets on the periphery of the wheel. The incoming light beam is reflected by a routing mirror or a routing beam splitter cube onto a front face of a facet as that facet is rotated in line with the routing element. The beam is deflected as it makes its first pass through the facet, and a porro prism redirects the deflected beam back through the rotating facet so that the beam makes two passes through each facet rotating past the routing element. The scanning action is produced by the rotation of the facet in the path of both passes of the beam through the facet. The net result cancels the angular motion of the beam in one direction and adds to the angular motion in the orthogonal direction. Scanning action as produced by one facet is repeated for each facet located on the periphery of the wheel.

I claim: 
     
       1. A scanning apparatus for repeatedly scanning a beam of light in a single same direction and from the same starting point, said apparatus comprising, light beam means for producing a collimated monochromatic light beam,   a prism wheel having a plurality of light deflecting prism facets on the periphery of the wheel and wherein all of the prism facets have substantially the same prism angle,   rotating means for rotating the wheel and the prism facets about an axis of rotation passing through the center of the wheel,   routing means for routing the light beam onto a front face of each prism facet as the wheel and the peripheral prism facets are rotated past the routing means, and   porro prism means for receiving the deflected beam exiting the back face of the prism facet and for reflecting the beam back through the prism facet so that the beam makes two passes through each prism facet rotating past the routing means and wherein the plurality of prism facets having the same prism angle are effective to produce a plurality of scanned beams each starting from the same starting point and each scanned in the same, single direction without retracing.   
     
     
       2. The invention defined in claim 1 wherein the routing means comprise a routing mirror effective to produce separation of the two passes of the beam through a facet. 
     
     
       3. The invention defined in claim 1 wherein the routing means comprise a beam splitter cube effective to produce overlap of the two passes of the beam through a facet. 
     
     
       4. The invention defined in claim 3 including polarization means for producing linearly polarized light at the inlet to the routing means and converting means for converting the polarization of the beam to an opposite polarization prior to the second pass of the beam through the beam splitter cube. 
     
     
       5. The invention defined in claim 1 wherein the wheel and prism facets are a one piece plastic molded structure. 
     
     
       6. The invention defined in claim 1 including a lens for focusing the beam after it makes its second pass through a facet. 
     
     
       7. The invention defined in claim 7 wherein the lens is a single meniscus lens located a distance away from the rotating prism facets and having curved surfaces chosen to minimize aberrations at the image plane. 
     
     
       8. The invention defined in claim 7 wherein the radii of the curved surfaces are chosen so that the beam passes nearby perpendicularly through the lens thickness. 
     
     
       9. The invention defined in claim 1 including servo control means for moving the routing means with respect to the direction of the incoming light beam in response to variations in the movement of an object on which the scanned beam is projected to thereby compensate for small variations in the movement of the object and to maintain the straightness of the scanned line on the object. 
     
     
       10. The invention defined in claim 1 including inlet means for directing the incoming beam into the routing means in a direction substantially perpendicular to the axis of rotation of the wheel and substantially in a plane containing the axis of rotation of the wheel so that the beam enters the routing means at a large entrance angle and at an entrance angle which minimizes the effect of the angle between the scanning beam and the normal to the routing means to produce a transmission relatively insensitive to scan angle. 
     
     
       11. A method of optimizing the straightness of a scanned line in an optical scanner of the kind having a rotating prism and routing means and reflecting means for causing a collimated monochromatic light beam to make two passes through the rotating prism and to produce the scanned line, said method comprising, (1) separately adjusting two of the angles between (a) the incoming beam and the wheel axis,   (b) the first face of the prism and the wheel axis, and   (c) the second face of the prism and the wheel axis      while holding one of the angles constant to achieve an optimum for each sequence of adjustment and to produce first, second and third optimums and   (2) then adjusting the angles to produce the better optimum of the three.